An HRSG is a heat exchanger to produce steam utilizing waste heat as heat source, and it is not only used for cooling, but also for providing waste heat to produce steam. Structurally, an HRSG may be classified as a shell-and-tube type, or a gas-pass type depending on whether the high temperature gas flows inside or outside a heat exchange tube. In a shell-and-tube HRSG, also called fire-tube HRSG, flue gas flows inside a heat exchange tube and water flows in a shell outside the heat exchange tube. Heat is transferred to water outside the tube through the tube wall. Generally, a shell-and-tube HRSG does not comprise a superheater section or an economizer section because water is simply evaporated. A large-sized shell-and-tube HRSG comprises a steam drum, a downcomer and a riser, while a small shell-and-tube HRSG uses the space in the top of the boiler shell to separate steam and water. In fact, a shell-and-tube HRSG is similar to the shell-and-tube heat exchanger in terms of structure, and also includes fixed tube sheet type, floating head type, U-shaped tube type. In a gas-pass HRSG, also called as water-tube HRSG, water or steam flows inside a heat exchange tube and flue gas flows in a gas pass outside the tube. The flue gas transfers heat to the water or steam in the tube through the heat exchange tube. Generally, a water-tube HRSG comprises a superheater section, an evaporator section, an economizer section, a steam drum, a downcomer, and a riser.
Raw syngas cooling is a commonly used process in fluidized bed coal gasification. The temperature of the raw syngas produced from a coal gasifier ranges from 850° C. to 1,000° C. and the raw syngas must go through dust removal and cooling processes before desulfurizing in the next stage in order to obtain clean syngas. In the prior art, the HRSG for fluidized bed coal gasification comprises a high temperature evaporator, a superheater, a low temperature evaporator, and an economizer, wherein the high temperature evaporator, the superheater, the low temperature evaporator, and the economizer all have the fire-tube structure. For example, a heat-tube HRSG with such a fire-tube boiler structure is disclosed in a Chinese utility model application with publication No. CN2426069.
However, due to the high contents of coal ashes and semi-cokes in the syngas produced from fluidized bed coal gasification, the fire-tube boiler inlets of the superheater, the low temperature evaporator and the economizer are bombarded by these coal ashes and semi-cokes, resulting in abrasion thereon, pipe leakage and system shutdown.
Further, because the existing high temperature evaporator is a vertically straight fire-tube boiler, gases and part of the steam produced by desalted water float to the top of the fire tube assembly of the high temperature evaporator, which will result in local overheating and damage of the tube sheet and the fire tube assembly.
Also, conventionally, the high temperature evaporator, superheater, low temperature evaporator, and economizer are separately located, and occupy a large space, making it difficult and troublesome to maintain to repair.
In the prior art, the economizer is made of carbon steel, which will result in dew corrosion on the outer surface of the steel pipe because the temperature in the economizer is not high enough.